The genus Streptomyces has been widely employed for industrial use as antibiotics-producing bacteria and has become an extremely important bacterial group for applied microbiology of today. In view of the significance of the production of useful substances by the genus Streptomyces, development of an inducible large-scale expression system capable of controlling the expression in actinomycete is desired. The present inventors have already obtained an inducible expression system for a protein (nitrilase) derived from Rhodococcus rhodochrous J1 (FERM BP-1478), which is one species of the actinomycete (JP-A-9-28380, and Proceedings of the National Academy of Sciences USA, October 1996, No. 93, pp. 10572-10577). This expression system expresses nitrilase (enzyme decomposing nitrile into acid and ammonia) in a large amount of not less than 35% of the total soluble protein in a cell-free extract, when isovaleronitrile is added to a medium as an inducer. In other words, an inducible promoter with an extremely potent transcription activity is involved in this expression system.
However, this finding concerns only a part of the actinomycete: Rhodococcus rhodochrous J1 strain, and there is no report or even a suggestion to date if a similar expression system can be utilized for actinomycetes belonging to other genera, particularly the genus Streptomyces, whose importance has been pointed out for the above-mentioned reasons.